1. Field of the Invention
The present invention relates generally to video displays and, more particularly, to video displays capable of real-time animation of video game scenes.
2. Description of the Prior Art
In addition to a color video display, video games include a digital computer, such as a microprocessor, whose operation generates the scene presented to one or more players on the color video display. The video game also includes one or more memories for digital data which contain the program executed by the digital computer and generally also image data. Execution of the program by the digital computer assembles the image data into a scene that then appears on the color video display. A player interacts to change the scene appearing on the video display through one or more controls such as push button switches a joystick, a track ball, a steering wheel, etc., also included in the video game. The video game signals from these various devices are supplied to the digital computer. In response to these signals, the digital computer alters the execution of its program and/or the selection and placement of image data within the scene. While there are several alternative techniques that may be employed for displaying a stored image on a color video display unit such as that included in a video game, at present the most widely used technique is called a bit map raster display.
A bit map raster display, is essentially what is done to display images on a conventional television screen. While not ordinarily visible to the human eye, the picture displayed on a television screen is made up of numerous, almost horizontal, parallel lines drawn across the screen's face beginning at the top of the screen and ending at its bottom. This arrangement of parallel lines is called a "raster." In standard television, the raster is made up of approximately 400 horizontal lines. Sixty times per second, an electronic circuit causes a beam of electrons to move from left to right along successive lines in this raster beginning at the top and ending at the bottom. The operation of this electron beam is called "scanning." As the electron beam moves along each line, another electronic circuit increases or decreases its intensity to brighten or darken a spot of light that the beam produces as it strikes the screen. Using this intensity modulation it is generally possible to resolve 512 or more individual locations along each such raster line as the electron beam scans its length.
To provide the signals required to display a scene on a television screen, a digital computer such as is used in a video game includes a digital data memory, generally referred to as a "bit map" memory or simply a "bit map," for storing scene data. To display a scene, this data is read from the bit map memory at the same rate as it is displayed on the television screen. Accordingly, the video game or similar device includes additional electronic circuits to synchronize the operation of the bit map memory and the scanning of the television screen, and to convert the digital data into signals suitable for changing the electron beam's intensity. Thus the operation of these additional electronic circuits uses the digital data retrieved from the bit map memory to present the viewer with the scene stored there.
In general, a scene to be viewed on a video display may be broken down into a background image plane such as the sky, a forest, a wall, etc. in front of which there is at least one foreground image plane. Each of these image planes may itself be further broken down into an array of picture elements generally referred to as "pixels".
In decomposing a scene into pixels for presentation on a video display, the pixels, which are generally all considered to be identically shaped rectangles, cover the entire area of the scene similar to tiles covering a floor. Further, throughout each pixel's area the scene has uniform color and color density. The area covered by an individual pixel depends upon the image's resolution. Thus, as a scene's resolution increases, the area covered by each pixel decreases while simultaneously the total number of pixels increases.
In general, the number of image planes making up a scene are greater than two. Thus, a scene will usually include a background image plane in front of which are several foreground image planes. In such a scene the planes are chosen so that no two of them intersect. This choice of image planes allows them to be ordered so each one is successively closer to the viewer. For example, a sky background image plane could lie behind a foreground image containing a cloud, and both the sky and the cloud image could lie behind a closer foreground image plane containing a character.
To properly reconstruct a scene that has been decomposed into a plurality of non-intersecting image planes, those planes are ordered so the background image plane is first, the image plane closest to the viewer is last, and the order of the intermediate image planes corresponds inversely with their distance from the viewer. After the image planes have been placed in the proper order, the composite scene may be assembled by superimposing successive image planes on top of each other. A set of transparent overlays illustrates this type of image reconstruction from an ordered set of image planes.
For a bit map raster scan video display, a set of digital data for the several image planes is employed. The digital data processing overwrites the data for a preceding plane image present in the bit map memory with the data for the next image plane closer to the viewer and so on for each image plane. Thus the composite scene produced by overwriting background image data present in a bit map memory with data for the foreground image plane next closer to the viewer is a new, composite scene which itself becomes the background image for all remaining images still closer to the viewer.
This technique has been recognized for some time. However, until now it has not been possible to use it in video displays, such as those used with video games and personal computers, for real-time display of scenes composed of several image planes. Real-time display of composite scenes was impossible because display processors used with the lower cost video displays have been unable to perform the digital data processing at the rates required for real-time television display.
For example, one earlier low cost display processor used in video games is capable of testing only one pixel of foreground data to determine if it is to be displayed. If a particular pixel of data is not to be displayed, this particular display processor employs a background generator circuit to display a pre-established, uniform background in that pixel. Thus, this display processor processes only a single plane of foreground data and displays a uniform background where there is no foreground data. Another prior video game display processor uses essentially the same technique but displays background image data read from a video disk.